The present invention relates generally to gas sensors and, more particularly, to carbon monoxide sensors.
Carbon monoxide sensors are used in a wide variety of applications, including the monitoring of heating installations that employ fossil fuels as an energy source and the monitoring of exhaust fumes from internal combustion engines. Two additional applications involve self-cleaning ovens and fuel cells.
Specifically, self-cleaning household ovens include a cleaning cycle that removes carbonaceous residues through a high temperature burning at a high power capacity for a fixed amount of time. Because the high temperature burning consumes a relatively large quantity of energy, there is a need for efficient self-cleaning cycles that automatically shut the oven off as soon as the burning process is complete. One way to accomplish this would be to monitor the carbon monoxide evolution during the heating cycle. Specifically, it is known that a typical dirty oven, when being cleaned at temperatures exceeding 800xc2x0 F., will begin to emit carbon monoxide at a temperature of about 550xc2x0 F. The amount of carbon monoxide being emitted will peak at around 800xc2x0 F. at a value of about 1500 ppm. After this peak value is reached, the carbon monoxide concentration decreases to around 200 ppm at the end of the cycle. An appropriate shut down point will occur for a typical household oven at a carbon monoxide concentration of about 200 ppm.
Therefore, there is a need for a robust, high-sensitivity carbon monoxide sensor to monitor self-cleaning oven cycles. The sensor must be sufficiently robust to withstand temperatures of 400xc2x0 F. or more. Further, the sensor must be sensitive enough to detect a relatively low concentration of carbon monoxide of less than 50 ppm.
Currently-available carbon monoxide sensors include infrared adsorption sensors and thin metal film metal oxide technology, such as tin oxide sensors. The infrared adsorption sensors are inappropriate for the household oven market due to their high cost and low sensitivity. The thin metal film oxide sensors are also inappropriate for use in monitoring self-cleaning oven cycles because they generally do not work well in a humid environment. Further, metal oxide sensors take a long time to regenerate.
Accordingly, there is a need for a low-cost, fast-response and high-sensitivity carbon monoxide sensor for use in self-cleaning ovens and other devices.
As noted above, another application for carbon monoxide sensors is in connection with fuel cells. Fuel cells are known devices that convert chemical energy of a fuel to electrical energy. Each fuel cell includes a pair of electrodes arranged across an electrolyte.
For proton exchange membrane fuel cells, the surface of one electrode is exposed to hydrogen or a hydrogen-containing gaseous fuel and the surface of the other electrode is exposed to an oxygen-containing, oxidizing gas. Electrical energy is produced at the electrodes through electrochemical reactions. Typically, a catalyst is used on the surface of the anode that is exposed to hydrogen or the hydrogen-containing gaseous fuel. One known problem associated with fuel cells is the deactivation of this catalyst by the adsorption of carbon monoxide which is present in trace amounts in hydrogen containing reformate fuel.
Therefore, there is also a need for a high sensitivity carbon monoxide sensor for monitoring the carbon monoxide level of the hydrogen or hydrogen-containing gaseous fuel fed to the anodes of a fuel cell. Again, existing infrared sensors are undesirable due to their high cost and low sensitivity and thin-film metal oxide sensors are disadvantageous due to their slow regeneration time and the instability in humidified environments. Thus, there is a need for a high sensitivity and economical carbon monoxide sensor for monitoring the concentration of carbon monoxide in the hydrogen-containing fuel fed to the anodes of a fuel cell.
The following summary of the invention is provided to facilitate an understanding of some of the innovative features unique to the present invention and is not intended to be a full description. A full appreciation of the various aspects of the invention can be gained by taking the entire specification, claims, drawings, and abstract as a whole.
In accordance with one aspect of the present invention, a carbon monoxide sensor comprises a sensing element that, in turn, comprises a catalyst dispersed over a metal oxide layer. The catalyst is capable of adsorbing carbon monoxide onto the metal oxide layer. The sensing element further comprises a temperature sensor. A flow sensor is provided for sensing the flow rate of gas directed at the sensing element. A signal processing module is coupled to both the flow sensor and the temperature sensor. The flow sensor sends signals indicative of the flow rate to the processing module and the sensing element sends signals indicative of the temperature of the sensing element to the processing module.
In a dependent refinement of this first aspect, after carbon monoxide has been adsorbed and accumulated onto the metal oxide layer, a heater heats the sensing element to a first temperature that is at least as high as the oxidation temperature of the carbon monoxide that has been adsorbed onto the first metal oxide layer, thereby resulting in an exothermic oxidation of the carbon monoxide adsorbed onto the first metal oxide layer and an increase in temperature of the sensing element to a second higher temperature. This temperature increase caused by the oxidation of the carbon monoxide provides an indication of the presence of carbon monoxide and, as discussed further below, in combination with the flow sensor signal, provides a basis for calculating the concentration of the carbon monoxide.
In other dependent refinements of this first aspect, the catalyst is selected from the group consisting of silver, gold, rhodium, ruthenium, palladium, iridium, platinum, metallic alloys of these elements and mixtures thereof. Presently, the preferred catalysts are platinum, ruthenium, ruthenium platinum alloys and ruthenium iridium alloys. The metal oxide layer can be a refractory metal oxide such as alumina, silica, titania and zirconia.
In further dependent refinements of this first aspect, a filter is provided upstream of the sensing element for purposes of keeping the sensing element clean and free of contaminants and foulants. Preferably, the filter is also disposed upstream of the flow rate sensor for purposes of protecting this component as well. In a further dependent refinement, the catalyst of the sensing element is a first catalyst and the metal oxide layer of the sensing element is a first metal oxide layer. In such a refinement, the sensing element can also include a protective second metal oxide layer disposed on top of the first catalyst and first oxide layer. The second metal oxide layer can be nickel oxide, zinc oxide or manganese dioxide. Other metal oxides will be suitable as well.
In accordance with another aspect of the present invention, a reference sensing element can also be provided as a part of the carbon monoxide sensor. The reference sensing element comprises the catalyst dispersed over a layer of the metal oxide. The reference sensing element also includes a heater and a temperature sensor. However, the reference sensing element is disposed downstream of a low temperature carbon monoxide oxidation catalyst that is dispersed on a substrate so that all gas that reaches the reference sensing element interacts with the low temperature carbon monoxide oxidation catalyst supported on the substrate. Accordingly, the low temperature carbon monoxide oxidation catalyst dispersed on the substrate is intended to oxidize all carbon monoxide before it reaches the reference sensing element. Thus, the reference temperature sensor of the reference sensing element provides a base line value which is communicated to the processing module. In a preferred embodiment, the catalyst of the reference sensing element is the same as the first catalyst and the oxide layer is the same as the first metal oxide layer.
In accordance with another aspect of the present invention, a method of detecting the presence of carbon monoxide in a gas is provided. The method includes exposing a catalyst dispersed on a metal oxide layer to a gas that comprises carbon monoxide. The catalyst is capable of adsorbing carbon monoxide onto the first metal oxide layer. The method also includes adsorbing at least a portion of the carbon monoxide in the gas onto the metal oxide layer below the carbon monoxide oxidation temperature. The method further includes heating the metal oxide layer to a first temperature that is at least as high as the oxidation temperature of the carbon monoxide adsorbed onto the metal oxide layer, thereby resulting in an exothermic oxidation of the carbon monoxide adsorbed onto the metal oxide layer and an increase in the temperature of the first metal oxide layer. The method further includes recording the temperature increase of the metal oxide layer, thereby indicating a presence of carbon monoxide in the gas.
In accordance with another aspect of the present invention, the aforenoted method further includes sensing a flow rate of gas that engages the catalyst and metal oxide layer over a fixed time period. Such a method also includes calculating a cumulative value for the carbon monoxide adsorbed onto the metal oxide layer from the increase in temperature of the metal oxide layer due to the exothermic oxidation of the carbon monoxide. Such a method would also include calculating a volume of gas by multiplying the flow rate by the fixed time period. The method also includes calculating the concentration of carbon monoxide in the gas by dividing the cumulative value by the volume.
In accordance with another aspect of the present invention, the aforenoted method can also include providing a reference cumulative value by providing a reference sensing element comprising the catalyst dispersed over a layer of the metal oxide and by providing a second catalyst supported on a substrate. The second catalyst and substrate would be disposed upstream of the reference sensing element so that it interacts with all gas that reaches the reference sensing element. The second catalyst would be a low temperature carbon monoxide catalyst which would thereby oxidize substantially all carbon monoxide in the gas that reaches the reference sensing element. The method would also include heating the reference sensing element to the first temperature simultaneously with the heating of the first sensing element, recording any temperature change of the reference sensing element from the first temperature, generating a reference value from the temperature change and reducing the cumulative value for the carbon monoxide detected by the first sensing element by subtracting the reference value.
The novel features of the present invention will become apparent to those of skill in the art upon examination of the following detailed description of the invention or can be learned by practice of the present invention. It should be understood, however, that the detailed description of the invention and the specific examples presented, while indicating certain embodiments of the present invention, are provided for illustration purposes only because various changes and modifications within the scope of the invention will become apparent to those of skill in the art from the detailed description of the invention and claims that follow.